Hi NatalieC,
This would depend on the type of cover that was available. With a mattress that had no zipper I would probably choose to have it glued to reduce the odds of shifting over time or with moving the mattress but with a zip cover I would choose loose layers so I could exchange single layers if that was necessary to fine tune comfort or support either initially or down the road or if one of the top layers needed to be replaced in the long term if it had softened sooner than the rest so I didn’t have to replace the whole mattress.
You can add legs under the center beam such as the examples in post #12 here to prevent it from sagging in the middle over time (if it doesn’t already have support legs to the floor). I would also make sure that the gaps between the slats were no more than 3" apart (and preferably less) and if the gaps were wider than that I would add more slats to the bed base to provide more even support and prevent the latex from sinking into the gaps over time.
Phoenix